Limited slip differential (assuming you weren't being cynical). AFAIK, no non-m BMW comes with a LSD (which are standard in sports cars. They all come with an open diff (meaning equal power distribution to both/all wheels) and attempt to compensate with DSC.
Personally, I was choosing between (seriously considering) 2008-2010 m6, 2009-2011 m3, and 2009-2011 335d. I chose the 2011 diesel (because I drive 40k+/yr) and I couldn't be happier with it. If choosing between the three as my DD, I would do it again. You just have to test drive and know what is right for you. Obviously it came down to the fact that a balance of performance and efficiency was more important to me that pure performance. I also enjoy modding, and am looking forward to doing a m3 suspension component conversion & LSD add to my diesel if I decide to keep it for the long haul.
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